A prime mover, such as an internal combustion engine or the like, can be connected to a hydrostatic transmission to drive a wheel in a light vehicle. A hydrostatic transmission is particularly suitable to provide traction drive for light vehicles such as turf machines, lawn tractors, ride-on lawn mowers, and like devices. A hydrostatic transmission may be connected to a variety of gearboxes and transaxles, so the same components can be utilized on a wide variety of light vehicle models. A simple usage of hydrostatic transmissions is on zero-turn radius vehicles, including zero-turn radius mowers and tractors.
In some vehicles, such as zero-turn-radius mowers, separate hydraulic pumps and motors are used to independently drive separate wheels of an axle. By independently driving the wheels in opposite directions, for example, the vehicle can be made to turn with zero radius. Zero-turn-radius mowers are increasingly popular as the size and cost of such mowers decrease. As the size of such mowers decreases, however, the space available for the hydraulic components and/or the prime mover also decreases.
Generally, a hydrostatic transmission includes a variable displacement hydraulic pump connected in a closed hydraulic circuit with a fixed or variable displacement hydraulic motor. The hydraulic pump may be a piston-type pump including a plurality of reciprocating pistons, which are in fluid communication through hydraulic porting with the hydraulic motor. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces hydraulic fluid through the hydraulic porting to the hydraulic motor to drive the motor, which allows the transmission output speed to be varied and controlled. The rotation of the hydraulic motor may be used to drive an output shaft, which in turn ultimately drives a wheel axle of a light vehicle of the types described above.
Hydraulic rotary actuators are used in a variety of applications in various industries. One use of a hydraulic rotary actuator is to control the rotational position of the swash plate associated with a hydraulic pump in a vehicle transmission of the type described above. Rotary actuators typically may include two chambers separated by a vane. By pressurizing the chamber on either size of the vane, the vane will rotate in the appropriate direction. For example, to rotate a vane clockwise, a chamber on a counterclockwise side of the vane is connected to a high pressure hydraulic fluid source, and a chamber on a clockwise side of the vane is connected to a low pressure source (or atmosphere). This pressure differential provides a force on the vane to rotate the vane in the clockwise direction. The vane assembly may be connected to an output drive shaft through which the rotational torque is transferred to an external device, such as for example the swash plate associated with a hydraulic pump as referenced above. Rotation in the counterclockwise direction is the same as the clockwise direction, except the clockwise side chamber is connected to high pressure and the counterclockwise side chamber is connected to low pressure (or atmosphere).
Precise control of the vane position, however, has proven to be a complex and difficult process. In conventional rotary actuators, control may be performed by precise electronic control of the fluid flow on the high pressure and low pressure sides. This often is achieved through complex and expensive electronic fluid control. Because of the difficulty of precise control, conventional rotary actuators have not been utilized to their maximum effectiveness, particularly in hydrostatic transmissions for zero-turn radius vehicles in which size and simplicity of design are of a particular concern.